1. Field of the Invention
This invention relates to packaging techniques for integrated circuits and, more particularly, to molded plastic packages in which a heat sink is included.
2. Prior Art
A heat sink is attached directly to a die-attach paddle of a lead frame using one of a number of conventional attachment techniques, including welding, soldering, brazing, and adhesive-bonding.
An example of an integrated-circuit package which uses a heat sink is shown in a U.S. Pat. No. 3,930,114, invented by R. Hodge and granted Dec. 30, 1975. The Hodge patent shows an integrated-circuit chip fixedly secured to the upper surface of a die-attachment pad. An elongated copper heat sink is brazed to the under side of the die-attachment pad. The Hodge patent provides a pair of L-shaped flexible fingers which forces the heat sink against the lower surface of a mold so that the lower, exposed surface of the heat sink is pressed against the surface of the mold and no molten plastic can penetrate onto the normally exposed surface of the heat sink. This eliminates the need to grind away hardened encapsulent material to expose the surface of the copper heat sink.
Using conventional attachment techniques to rigidly attach a heat sink to a lead frame for an integrated circuit may raise several problems, particularly if dissimilar materials are used. One of these problems is the dissimilarity in the temperature coefficients of expansion (TCE) for the different materials used in the lead frame, the heat sink, and sometimes the attachment material. These differences in the temperature coefficients of expansion are important because of the possible reliability problems caused when stresses are built up in a package assembly at various temperature due to the differences in the TCE's. Restrictions on selection of material to avoid dissimilarities in TCE's may preclude using the best materials possible for particular functions. For example, copper or a copper alloy is a good material for a lead frame and aluminum or an aluminum alloy is a good material for a heat sink. The TCE's (in units of 10.sup.-7 /degree Centigrade) for various materials are: copper: 170; Cu/W alloy: 70; Cu/Mo: 72; and aluminum: 230.
Another problem is the extra cost incurred in using special tooling and special materials for a package with a heat sink, as compared to the standard cost incurred in using a standard package without heat sinks. For example, a package mold designed for use with an aluminum alloy leadframe cannot be used with copper lead frames. High tensile strength copper is typically more expensive than A142 aluminum alloy or aluminum.
Another cost item is that heat sinks must be prefabricated and attached to the lead frame at the lead frame manufacturing site. This increases the need for additional inventories of lead frames with heat sinks and lead frames without heat sinks at an assembly sites to accommodate needs for both types of lead frames. This also requires that additional raw materials be stocked at a lead frame manufacturing sites.
Various types of drop-in or floating heatsink designs have addressed the problems associated with the inventory problems and problems associated with dissimilarities in the temperature coefficients of expansion (TCE) for the different materials. These drop-in heatsinks sometimes fail to provide positive thermal coupling between a lead frame and a heatsink. Too large of a gap between the drop-in heatsink and the lead frame allows molding compound to fill the space between the lead frame and the heatsink. Because the molding compound is a thermal insulator, the thermal efficiency of the package can be affected. Too small of a gap between the drop-in heatsink and the lead frame can create a trapped air space. The trapped air space is also a thermal insulator and can be a source of unreliability because moisture tends to gather in the trapped air pocket. An additional concern with a drop-in or floating heatsink design is called pad tilting. Molding compound injected into a mold cavity at a high velocity can differentially fill the gap between the heatsink and the lead frame. This creates pad tilt which creates a reliability problem for plastic quad packages with high pin counts.
The need has arisen for an economical integrated-circuit packaging technique which provides a standardized lead frame which can be mechanically isolated from an optionally used heat sink.